Many airplanes have high-speed electrical generators that are used for generating power during flight. The electrical generators generate AC power, which is converted to DC power. The DC power is then supplied through a DC distribution system to on-board electronics such as radar, vapor cycle compressors, flight control electronics, electromechanical/electro-hydrostatic actuators, and the like. The electrical generators can be wound field synchronous machines, switched reluctance machines, permanent magnet machines, or other types of machines.
Conventional wound field generators can provide short circuit protection. However, conventional wound field generators are incompatible with high-speed prime movers unless a gear reduction stage is added. Unfortunately, the gear reduction stage adds cost and complexity to the airplane design.
For high-speed applications, the permanent magnet machine is desirable because of its robust rotor design and a low magnetic spring rate associated with its large air gap. However, the permanent magnet generator's excitation is fixed which does not provide for a “graceful survival” of short circuit conditions.
For example, because of their low impedance, short circuit currents in excess of the permanent magnet generator's current rating can flow, causing excessive heat build up in the generator's stator windings. A short at the terminals of the machine, or at the DC link can literally melt the windings and destroy the generator. Thus, if a generator cannot survive the short circuit conditions, the generator cannot recover and deliver power to the aircraft when the short circuit is removed.
There is a need for a high-speed permanent magnet generator that can gracefully survive short circuit conditions.